The present invention concerns a novel polymeric alumino-silicate and a method for preparing it.
Polymeric alumino-silicates in a fibrous form are known. A fibrous, tubular crystallized alumino-silicate known as imogolite is present in the impure natural state in volcanic ash and in certain soils.
U.S. Pat. Nos. 4,152,404 and 4,252,779 of Farmer describe an inorganic material similar to natural imogolite. This inorganic material is synthesized by causing silica or a soluble silicate to react with an aluminium compound so as to form a complex hydroxyaluminium silicate in aqueous solution with a pH of 3.2-5.5, and then effecting a digestion of this complex at a pH of 3.1-5.0 so as to form a colloidal dispersion of the inorganic material. This inorganic material can be used as a molecular sieve, a catalyst support, a coagulant or an adsorbent. By evaporating the colloidal solution of imogolite on a flat surface, it is possible to form films which can be used as membranes. If the material is not isolated from its colloidal solution, it can be used as a flocculant, a substance for promoting hydrophilicity or a thickening agent.
European Patent 0 250 154 describes a photographic product, comprising a polymer surface on which a layer comprising a gelled lattice of inorganic particles, preferably oxide particles, has been caused to adhere. According to this patent, the gelled lattice forms a porous layer with voids between the inorganic oxide particles. This porous layer is obtained from a dispersion or suspension of finely divided particles in a liquid medium. The oxide particles can be boehmite (aluminium oxide), silica or a silica gel coated with alumina. This gelled lattice is formed by the aggregation of colloidal particles bonded together in order to form a porous three dimensional lattice. This gelled lattice provides a substratum endowed with antistatic properties.
European Patent Application 0 741 668 describes a homogeneous polymeric alumino-silicate having antistatic properties, as well as a method for obtaining this alumino-silicate with a high degree of purity. According to European Patent Application 0 741 668, the method for obtaining the polymeric alumino-silicate comprises:
(a) the treatment of a mixed aluminium and silicon alkoxide with an aqueous alkali at a pH in the range of 4 to 6.5, maintaining the Al molar concentration between 5xc3x9710xe2x88x924 and 10xe2x88x922 M and the Al/Si molar ratio between 1 and 3;
(b) the heating of the mixture obtained from step (a) to a temperature below 100xc2x0 C. in the presence of silanol groups, for a sufficient period for obtaining a complete reaction and the formation of an inorganic polymer, and
(c) the elimination of the residual ions from the reaction medium.
The inorganic polymer is a fibrous alumino-silicate of formula AlxSiyOz where x is in the range of from 1 to 3 and z is in the range of from 1 to 10.
The object of the present invention is a polymeric alumino silicate, derived from the polymeric alumino-silicate of aforementioned Patent Application 0 741 668, and a method for obtaining this derived material.
The material of the invention is a polymeric alumino-silicate material of formula AlxSiyOz, in which x:y is a number from 1 to 3 and z is a number from 1 to 10, this material being substantially comprised of spindles with a length L of between 10 and 100 xcexcm, a maximum width 1 of between 2 and 20 xcexcm, the ratio L:1 being a number from 3 to 10.
Preferably, x:y is a number from 1.5 to 2.5, z a number from 2 to 6, L is between 20 and 80 xcexcm and 1 is between 5 and 15 xcexcm.
The terms xe2x80x9csubstantially comprisedxe2x80x9d mean that the material comprises at least 95 weight % and preferably at least 99 weight % of said alumino silicate spindles, based in the total weight of the material.
The method for obtaining the material of the invention comprises the steps of:
(a) treating a mixture of an aluminium compound and a silicon compound, both hydrolysable, or a hydrolysable mixed compound of aluminium and silicon, by an aqueous alkali, at a pH in the range of 4 to 6.5, maintaining the Al concentration between 5xc3x9710xe2x88x924 M and 10xe2x88x922 M and the Al:Si molar ratio between 1 and 3;
(b) heating the mixture obtained from (a) at a temperature below 100xc2x0 C. in the presence of silanol groups, for a period sufficient for obtaining, by means of a complete reaction, the formation of a polymeric alumino-silicate in solution;
(c) concentrating the solution obtained from (b) so as to obtain an Al+Si concentration of at least approximately 1.5 g/l,
(d) settling the concentrated solution obtained from step (c) to produce 2 phases, and collecting the upper (less dense) phase, which represents at least 90% and preferably at least 95% by weight polymeric alumino-silicate with a spindle morphology.